Cleaning of metallic surfaces



bVl-I VVIIII uulllvlvl "T" Sept. 5, 1939. w. HEIMBERGER 2,171,931

CLEANING PF METALLIC SURFACES I Filed Nov. 20, 1937 4 Sheets-Sheet 2 UlU00 llUiUlUllUV Sept. 5, 1939. 1 w, H ER 2,171,981

CLEANING 0F METALLIC SURFACES Filed Nov. 20, 1937 4 Sheets-Sheet 3 Liai-1 2 3 5 is i '6 cams/M1011 "/m T ig El I I I I Jrzwvz/ok 1 2 3 s e zccnsMO/f 6 10 UIVUOO HUIUIUHUU Filed Nov. 20, 1937 4 Sheets-Sheet 4Patented Sept. 5, 1939 umrso STATES PATENT OFFICE Application November20, 1937, Serial No. 175,623 In Germany October 23, I934 10 Claim.

This application isa continuation-in-part of my application for U. 8.letters Patent Serial No. 14,801, filed April 5, 1935, for the Cleaningof metallic surfaces.

The present invention relates to the cleaning of metallic surfaces bypickling-and more especially to the pickling of iron. It is concernedwith the treatment of metals such as iron preparatory to the coating ofthe metal surface with enamel or paint or another metal. It is an objectof my invention to render the pickling process simpler and cheaper andless dangerous for the operator and to improve the quality of thepickled product.

Iron in the form of sheet or other roller products is not as a ruleresistant against the action of the atmosphere or of acids. In order toproduce stability, the surface of the metal must be efllclentlyprotected'by painting or enameling or metallizing or like? measures. Anefllcient protection is however'obtained only, if the protective coatingis applied'to the clean metal.

In order to obtain clean iron surfaces, it is usual to remove adheringscale, rust or impurities 2'5 by means of acid solutions. Thesesolutions, the

so-called pickling baths, consist as a rule of sulfuric acidorhyd'rochloric acid with a concentration of some per cent, for instance10%.

It is further known that the pickling process =so-may be accelerated byan addition to the acids of salts which sometimes also purport toprotect the bare. metal against the attack of the acid during thepickling process.

Thus it has for instance been proposed to treat stainless steel-alloysfor the removal of scale with a solution containing a mineral acid andan alkali metal nitrate. The preferred concentration of both the acidand the nitrate was from about I to 5%, while :.the content even variedfrom 40 0.2% to about 40%.

, All these known pickling baths involve certain drawbacks whichhitherto were considered necessary evils resulting from theconcentration required fpr'an' eihcient pickling. Thus operation withrelatively high acid concentration involves a certain danger to theoperator and the acid vapors may even render the atmosphere almostunrespirable. The hydrogen developed may render the iron to he pickledbrittle. An important 59 drawback is the fact that the pickling baths Vgradually become richer with dissolved iron salts to such an extent thatthey can no longer be used whena clean surface is intended to beobtained by the pickling process. Perfectly clean ainscescannothevbtimda a l si i manner, but the blank under treatment must afterwards bescoured with sand and water in order to enable a satisfactory enamelcoating to be produced thereon.

According to the present invention these draw- 5 backs are obviated,while the pickling process as such is at the same time rendered simplerand less expensive. I use principles hitherto not employed in theart ofpickling by utilizing knowledge gained from the colloidal chemistry. Icon- 1 vert the bi-valent iron salts formed in the pickling operation,by an addition to the pickling bath of oxidizing agents, into tri-valentsoluble iron salts, which are then caused to partly change intocolloidal ferric hydroxide or other 15 basic ferric compounds bysuillciently diluting and heating the bath.

Several conditions must be fulfilled in order that this process can takeplace continuously. In the first line the acid strength must be verylow, go much lower than hitherto used for the pickling operation. Theacid strength must be low enough to allow the formation of basic ferricsalts by hydrolytic action. I have found that for this purpose theconcentration of the acid 5 during the operation proper must not exceed0.1%, since insoluble iron compounds 'do not occur before theconcentration of the acid has dropped to or below this value. The acidconcentration must be the lower. the lower the tema0 perature of thepickling bath. I have further found that the content of the picklingbath at the beginning of the operation should not exceed about 1%sulfuric acid.

According to my invention it .is a further con- 85 dition for asuccessful operation to maintain the pickling bath at a temperatureabove0., since the reaction mentioned which leads to the formation ofinsoluble basic iron salts, does not proceed appreciably below thistemperature. I 40 prefer to heat the pickling baths to about 70-80 C.The higher the temperature, the higher is the acid strength .at whichthe insoluble iron salts start precipitating.

The formation of insoluble ferric hydroxide or 45 other insoluble basicferricsalts in the form of colloids as attained under theseconditions-of concentration and temperature does not yet secure anyparticular advantage. I therefore provide for the presence, in thepickling bath, of a 50 suitable electrolyte capable of coagulating thesecolloidal iron compounds in order to precipitate them from the solutionin the form of a slurry, which may be filtered off. An electrolyte maygenerally be considered suitable for this purpose, if its anion differsfrom that of the pickling acid and its kation is less noble, in thesense of the electromotive series, than the metal to be pickled.

' An oxidant must be present in the pickling bath for converting theferrous .ions into ferric ions. The electrolyte which serves tocoagulate or precipitate the colloidal iron compounds, may be'identicalwith the oxidant and may for instance be an alkali metal nitrate.

In the practice of my invention satisfactory results are obtained if thepickling solution at the beginning of the operation shows for instancone of the following compositionsz 1% sulfuric acid+2% potassiumnitrate, or

1% nitric acid+2% magnesium sulfate, or 2% potassium persulfate+2%sodium nitrate.

I obtain good results also with other solutions, for instance with amixture of nitric acid and sulfuric acid with magnesium sulfate, but Ihave found it advantageous inany case to have sulfate ions in thepickling bath, and this may be connected with the formation in the bathof F2(SO4)2O as insoluble ferric compound, as will presently beexplained more in detail with reference to the drawings afflxed to thisspecification and forming part thereof.

In the drawings: Fig. l is a diagram showing the pH values of a sulfuricacid solution of different acid concentrations.

Fig. 2 shows diagrammatically the concentration of sulfuric acid atwhich, at different temperatures, basic iron salts start precipitatingso that the solution will become turbid, while Figs. 3 to 10 showdiagrammatically curves obtained by titrating baths with caustic sodasolution, as will be explained more in detail farther 40 below. 1

First, however, I will develop the fundamental reactions on which Iassume the new process is based and which may render intelligible theadvantages attained.

If a piece of sheet metal or another blank is placed in a pickling bathaccording to my invention, i. e., a bath containing a suitableelectrolyte, an oxidizing agent and a mineral acid of a strength notexceeding about 1%, this bath being heated to at least 50 0., part ofthe acid is consumed in the formation of iron salts. Under the action ofthe oxidizing agent these salts change over into colloidal ferrichydroxide, as soon as the acid concentration has dropped to a value atwhich the formation of colloidal ferric gyioxide is possible at thetemperature of the While it cannot be ascertained with certainty,whether ferric hydroxide as such is precipitated 00 immediately or forinstance a basic salt like F82(SO4)2O, the following equilibrium may beasumed to prevail in the bath during the pickling process according tomy invention:

In the absence of metallic iron the insoluble or coagulated basic ferriccompound would precipitate by hydrolytic action, until the solubilityproduct is reached as indicated by the law of mass sulfate is consumedin a reaction, the equation of which may be assumed to be action. In thepresence of iron, however, the. sulfuric acid formed by the hydrolysisof ferric ceed from the left hand side to the right hand side ofEquation 1. It is true that the ferric sulfate of the left hand side ofthis equation may also react with the metallic iron present, forinstance according to the equation 2Fe+2Fez(SO)a- 6Fe80 In but theferrous sulfate thus formed changes, in

consequence of the oxidizing agent present, into ferric sulfateaccording to the equation 6FeS04+3H2S04+3O- 3Fe:(SO;) 3+3H2O IV From theEquations III and IV there results 2Fe +3H2SO4+30- Fe2 (S04) a+3H2O V beassumed to take place in the pickling bath:

.The principle of this process would not be changed, if in realityferric hydroxide itself or another basic ferric compound were formed andprecipitated, instead of Fe:(SO4):O. In any case the reactions, by whichthe iron is pickledaccordingto my invention, differ greatly from theprocess according to which pickling is hitherto carried out and whichmay be formulated as follows:

2Fe+3HzSO4+3O- Fez(SO4)3+3Ha0 VII and according to which no ironcompound is precipitated, while the bath is continuously enriched withsoluble iron salts.

When according to my invention colloidal ferric hydroxide is formed andcoagulated or some other insoluble basic ferric compound is produced byhydrolysis, the anion of the dissolved, tri-valent iron salt iscompletely 'or partly set free, as may be guessed from the aboveequations, and it is capable of again participating, in the form ofacid, in pickling reactions. A fresh portion of iron may therefore bedissolved. Since part of the colloidal ferric hydroxide is continuousl'yprecipitated as insoluble matter under the action of the electrolytepresent, shortly after the begining of the pickling process, i. e.,after the blank has ben placed in the bath, a chemical equilibrium isestablished which shows the peculiarity that the concentration of thedissolved iron is kept very low and practically constant.

In the course of my investigations I have made a great number ofexperiments to ascertain the particular features of the new picklingprocess and I will now describe someof these experiments more in detailin order yto facilitate a complete understanding of my invention.

I have carried out a series of tests in which samples of scaled ironsheet were pickled-with solutions of sulfuric acid and sodium nitrate ofvarious concentrations and at various temperatures. crystallized sodiumnitrate of commerce and sulfuric .acid of commerce wereused, whichlatter was diluted with tap water and adusted, by titration with n NnOH(pH values) with a foil colorimeter according to Wulfl, by means ofwhich the pH values from 1.6 upwards may be ascertained with sufficientaccuracy. The pH values were reduced to per cents of acid by means ofthe curve of Fig. 1 which is drawn according to the data published byKordatzki.

The state of the pickling solutions may be ascertained by establishingtitrating curves which fairly accurately show the quantities ofbi-valent and tri-valent iron compounds present in the solution. Suchtitrating curves are established in the following manner: the startingpoint of the curve is represented by the pH value of the solutionitself, the further values of the curve are obtained by gradually addingto ccrns. of the solution small quantities of a solution of causticsoda, determining the pH value after every such addition and plottingthe values thus obtained as points of the curve. The course of thetitrating curves is of interest for my purpose only up to the point ofneutrality, i. e., pH=7. An acid solution which contains bi-valent andtri-valent iron salts, shows a titrating curve which has two well markedbreaks-at about pH=3.0 and pH=6.3, each of which is followed by asubstantially horizontal part of the curve. At the pH value of about 3.0ferric hydroxide precipitates, while at the pH value of about 6.3 fer.-rous hydroxide is precipitated. The length of the horizontally extendingpart of the curve which follows these values, allows to guess thequantity of iron ionsof the corresponding valence present; thehorizontal part of the curve is the longer, the more iron of thecorresponding valence is dissolved.

The tests of this series were carried out in beakers with 200 ccms.solution. The heating, if

any, of the solution was performed in a water bath, During theindividual tests the samples were movedfrom time to time.

The solutions were mixed in cold state. The pH value of the startingsolution was drawn from the curve of Fig. 1.

In Test No. 1 the influence of an aqueous solution containing 0.1%H2SO4+2% NaNOs on black plate was investigated at 20 C. This solutionwas rendered turbid by the formation of insoluble ferric compounds onlywhen ferric ions could form and when the soluton was neutralized to suchan extent that Fe( H): could form. The titrating curves showed that thisstate is attained at a pH value of 2.9 to 3.1, corresponding to aboutBoth the pickling effect of the cold solution on scale and the oxidimngefiect of the sodium nitrate added were very small and a cleanironsurface could not be attained with certainty. The titrations showthat only very small quantities of iron compounds were formed.

Test No. 2 was carried out with the same concentrations, but at 65 C. Inconsequence of the higher temperature the time required for pickling wasshortened, but the velocity of pickling was still rather low, due to thelow acid strength of the bath at the start. The solution became turbidat pH=2.3-to 2.5 (corresponding to 0.03 to 0.02% H2804) or, if heated toboiling, at pH=1.9 to 2.0 (0.07 to 0.06% H2804) In the course of thistest the diagram shown 10 lution containing 0.3% H2SO4+2% NaNO3. The

pickling velocity was about the same as in Test No. 1. Only smallquantities of iron compounds were formed in the solution, as could beseen from the titrating curve. In spite thereof the sodium nitrate addedneeded several days for converting the dissolved ferrous compounds intoferric compounds. These ferric compounds are however not present in theform of colloidal Fe(OH) 3, since they proved to be easily reduced whenthe sheet metal was placed again in the solution. The solution becameturbid, when the pH value reached 3.1. The clean-pickling effect of thesolution was satisfactory.

Test No. 4, being very instructive, shall be reproduced in detail:

The solution contained 0.3% H:SO4+2% NaNOa. The temperature was 65 C.pH=-.1.3.

Time-of observation 2:00p. m. The sheet was placed in the solution.Developing gas could distinctly be observed.

2:04 p. m. The sheet was wiped, the scale showed to be already dissolvedto a great extent. When caustic soda solution was added to a sample ofthe solution, a green precipitate was formed. (Such precipitatingreactions were repeatedly performed in samples of the solution in orderto investigate the state of the solution, and I will shortly indicatethe color of thissample-precipitate in the following explanations.)pH=2.3. The solution had a light yellowish color.

2 :22 p. m. The sample-precipitate brown.

was dark 2:30 p. m. The sample-precipitate was brown.

2:32 p. m. The sheet was again placed in the solution, a slightdevelopment of gas could be observed. 2:36 p. m. The sheet was wiped; alight brown covering had formed on the plate, which could easily bewiped ofl. The sheet was perfectly clean save only small spots of scale.

2:40 p. m. The sample-precipitate was dark brown. pH=2.3, color of thesolution distinctly yellow.

Tlmeof observation 2:45 p. m. The sheet was again placed in thesolution. A covering formed on the sheet, which was rinsed off when thesheet was moved. Below this covering however a light, adherent covering,the formation of which is very characteristic for my pickling process,was gradually produced. I 2:49 p. m. The sheet was wiped and after thistreatment of about 12 minutes showed to be perfectly clean. The solutionis dark yellow colored] and slightly turbid. Small quantities of aprecipitate are present. p1'-L=2. 4.

3:00 p. m. A fresh plate was introduced. -A

rather thick covering settled on it. but was rinsed off when the sheet Iwas moved. I 3:04 p. m. The sheet was wiped- Only a very small part ofthe scale was dissolved. The solution was distinctly turbid.

3:06 p. m. The sample-precipitate was dark brown. pH=2.5. 2 ccms. H2804(containing 100 grams H2804 per liter) were added; the solution be.-

came somewhat lighter.

3:14 p.m. The solution was not yet perfectly clear again. The sheet wasagain placed in the solution, a very slight development of gas could beobserved. I

3:1 I m. The sheet was wiped and the scale was I found to be removed toa consid- I erable extent. At the places, which were free from scalealready before the sheet was placed in the solution, a brown coveringhad formed. The solution was some what more turbid.

3:20 p.m. The sample-precipitate was dark brown to olive-colored.pH='2.4. 2 ccms. H2804 were added.

3:28 p. m. The sample-precipitate was dark brown to olive-colored. Thesolution was. greyish brownand dis- I I tinctly turbid.

:31 ptm. The sheet was again placed in posi- I tion. A brown coveringformed.

3:34 p. m. The sheet was wiped. Only small remnants of scale werepresent. The solution was more turbid and of darker color. I 3:38 p. m.The sample-precipitate was dark olive-green. -pH=2.5. 2. ccms. H2804were added.

-- 3:48 p.m. The sample-precipitate was dark olivesgreen. pH=2.3. 2H2804 were added. 4:02 p. m. The turbidity of the solution haddisappeared. .The solution was brown-olive-colored. Thesampleprecipitate was dark brown-olivecolored. pH=2.1. 2ccms. 8:804

- were added. I

4:28 p. m. The sample-precipitate was dark 4 brown; pH= 1.9.

:33 p.m.. The sheet was introduced into thebath, a slight development ofgas could be observed. I

Timeoi" observation I I I 4:36 p. m. The sheet was wiped. The lastremnants of scale had disappeared, the sheet was perfectly clean after 5this treatment of abqut 14 minutes.

4:41pm. The sample-precipitate was dark brown olive-colored. pH'=1.8.

4:45 p. m. A fresh plate was placed in the solution. A light coveringwas formed 10 which could be rinsed off.

4:49 p. m. The sheet was wiped, about half of the scale was found to beremoved. The solution appeared slightly turbid. 4:51p. in. Thesample-precipitate was colored dark olive-green. pH=1.8.

5:02 p. m. The sample-precipitate was dark olive-green. The sheet wasagain placed in the solution. A brown'20 covering formed. 5:06 p. m. Thesheet was wiped, the scale showed to be not completely re- I moved. J

5:15 p. m. The sample-precipitate was dark olive-green. pH=2.2. Thesolution became turbid and a 'flocculent precipitate appeared.

5:21 p. m. The sheet was again placed in the solution.

:25 p. m. The sheet was wiped and, after this treatment of about 12minutes, was found to be perfectly clean save for a few small spots ofscale. The solution was very turbid, its color greyish brown.

5:37 p. m. The sample-precipitate was almost a pure green. pH=2.3..

The solution was left standing about 42 hours. v

light yellow. Aflocculent ocherous precipitate was present. 4 gramsNaNO: were added, the solution was not heated.

Third dai! Time of observation :1 a. m. The sample-precipitate wasgreen.

0 p. m. The sample-precipitate was green.

The solution was heated.

:05 p. m. The sample-precipitate was green.

4:30 p. m. The sample-precipitate was green.

pH=2.3. 2 ccms. E804 were 00 added. The precipitate was not dissolved.

5:00 p. m. The sample-precipitate was green.

pH=2.8. 2 ccms. H3804 were The solution was left overnight and allowedto 1 cool down.

added. The precipitate was not m :43 a. m.

a Fourth day Time of observation 8:30 a. m. The sample-precipitate wasdark olive-green. pH=1.7. The precipitate in the solution was dissolvedsave for a small residue. The cold solution was light yellowish colored,the color changed to light brown on heating.

The titrating curve illustrated by Fig.

3 was established. V The sample-precipitate was brown olive-colored. 4grams NaNO: were added. The titrating curve illustrated in Fig. 4 wasestablished.

The color of the solution was brown olive. The solution wascontinuouslyheated.

The color of the solution had become .light brown. Thesample-precipitate was red-brown. The titrating curve shown in Fig. 5was now established.

A fresh black plate was placed in the solution.

found-to be only wry little attacked. 2 ccms. H1804 were added. Thesample-precipitate was brownolive. pH=.-1.7.'

The sheet was again placed in the solution. v

The sheet was wiped, nearly the half of the scale was found to beremoved. The color of the solution was greyish-brown.

The sample-precipitate was dark brown.

p. m. The titrating curve shown in Fig. 6

was established.

The'soluticn was left over to cool down.

Fifth day night and allowed Time of observation 9 :os a. m. The sheetwas again introduced into the heated solution. A covering was formed.

The sheet was wiped, the scale proved to be farther dissolved.

The sample-precipitate was redbrown. pH=2.3. The pickling bath was ingood condition.

The sheet was again placed in the solution, a covering occurred.-

The sheet was wiped and showed only some stripes of scale. The sheet wasagain placed in the solution. A strong covering formed. The sheet waswiped. The. scale showed to be still further dissolved.

The sheet was again introduced in the solution and a strong coveringformed.

9:11 a.in.

9:24 a. In.

i am a. m. I

treatment of about 21 minutes, showed to be perfectly clean save forsmall spot of scale.

The sheet was wiped, the scale was The results of Test No. 4 may besummarized as follows:

The pickling velocity was greater than in Test No. 2, but it wasconsiderably reduced by a further addition of sodium nitrate. Thepickling effeet of the solution is good and secures clean surfaces.

At the beginning of the test a sheet metal was first immersed into thesolution for a short time and the solution was then allowed to standuntil the ferrous compounds, which had first formed, were converted,under the oxidizing action of the sodium nitrate, into ferric compounds.The oxio'iz.ng effect of the sodium nitrate is only small, probablybecause of the small starting concentration of the acid. During thefirst part of the test apparently no colloids were formed, as may beconcluded from the fact that the ferric compounds could always easily bereduced. Additions of acid during the course of the test are of noinfluence in this respect. An addition of sodium mtrate eifected averyslow oxidation, the course of which may be seen from the successivetitrat- I in'g curves shown in Figs. 3 to 7.

An increase of the content of sodium nitrate A The sheet was wiped and,after this The sample-precipitate was 'red-.

during the test up to 6% caused the precipitate, which was formed duringthe first part of the test, but was dissolved by an addition of acid, tonot be formed afresh, although the pH value at the end of the test wasagain 2.3 and only ferric compounds were present in the solution. Thetitrating curves show also, that no precipitate wasformed; for thecontent of dissolved iron compoundsin the solution is shown by thesecurves Y to be always about the same, A nitrate concentration of 6% isthus too high.

The slight brown color of the solution in the second part of the testcannot be attributed to theformation of colloids. since the increasedcontent of sodium nitrate prevents the formation of colloids. The browncolor may indicate the formation of Fe(NO1):. 1

Test No. 5 was carried through with the same starting concentrations asTest No. 4, but at 80 C. The clean-pickling eifect of the solution issatisfactory. The pickling velocity is increased due to the highertemperature, but a further std--v dit.on of sodium nitrate proved againto reduce the pickling velocity. The colloids which could form in thesolution when it contained 2% NaNOa, disappeared almost completely, whenthe content was increased to 4%. An addition to the pick ling bath of 4%NaNOa may thus be considered as the highest quantity admissible at apickling temperature of 80 C. The specific consumption of acid,calculated on the surface of the pickled sheet, only amounts to aboutone half of that found in Test No. 4. I

The eifect of the presencein the solution of ferric ions may beillustrated by two further tests. numbered No. 8 and No. 9, in both ofwhich the pickling bath contained 0.7% HaSO4+2% NaNOa, giving a startingpH value of 1.02, while the temperaturewas C.

Tlsr No. 8 Time of observation 3:17 p. m. The sheet.was placed in thesolution. A vigorous development of gas occurred, which became graduallystronger. 4

3 :21 p. m. The sheet was wiped, the scale showed to be dissolved almostcompletely.

3:22 p.m. The sheet was again introduced.

' The development of gas was considerably reduced, but became strongerwhen the sheet was moved.

3:26 p.m. The sheet was wiped and, after this 13:44pm. The sheet waswiped. It showed to be free from scale, but not clean at all; at thefree air it tarnished with greyish-green color.

' 3:47 p. m. The sheet was introduced again.

3:51 p. m. The sheet, on which a very light grey covering had formed,was wiped and showed to now be well cleaned after this treatment ofabout 12' 3:56 p. m. The sample-precipitate was green.

' pH=1.6. The color of the solution was a light olive-green.

fresh sheet was introduced. A hardly appreciable development of gasoccurred.

4:05 p. m. The sheet was wiped. The main part of the scale showed to beremoved.

4:06 p. m. The sheet was again introduced into the solution. After a fewminutes a very slight brown covering was formed. I

4:10 p. m. The sheet was wiped. v Thescale was not completely dissolved,the parts 4:01 p.m. A

which were free from scale, were 5 also clean. 4:13 p. m. Thesample-precipitate was green.

pH=2.3 The solution was clear,

- its color olive-green.

4:15 p.m. The sheet was again introduced and a slight development of gasoccurred. A weak covering formed on both sides. I

4:25 p. m. The sheet was wiped, the scale showed to be not yetcompletely dissolved.

4:26 p.m. The sample-precipitate was green. pH=2.3. The solution wasstill perfectly clear.

4:31 p. m. The sheet was again placed in the solution. A fairly uniformcovering formed on both sides.

4:35 p.m. The sheet was wiped and, after this treatment of about 24minutes,- showed to be not perfectly freed from scale while the partsfree from scale were also substantially clean. The pickling eflect ofthe solution seems to be exhausted.

4:38 p.m. The titrating curve shown in Fig. 8

was established.

5:05 p.m. The solution became turbid. The

sample-precipitate w a s g r e e n pH=2.3-

( Tlmeof- 1 TmrNoJ' observation Time of of gas occurred (this sample ofsheet seemed to be somewhat less oxidized than the other ones).

5:28 p. m. The sheet. was wiped and, after this treatment of about 3minutes, showed to be free from scale and fairly clean.

5:32 p.m. The sample-precipitate was green.

The solution was allowed to stand, while maintained at 65 C.. until thesample-precipitate became brown. 5:50 p.m. The sample-precipitate wasdark brown. I 6:00 p.m. The sample-precipitate was brown. The solutionwas left standing over night and allowed to cool down.

Second day Time of observation 8:30 a. m. A fresh sheet was introduced.A

slight development of gas occurred."

8:50 a. m. The sheet was wiped and. was found to be perfectly clean savefor a few spots. The solutionwas yellowishbrown.

8:52 a. m. The sample-precipitate was red- 'brown. pH=1.7.

8:59 a. m. The sheet was introduced again. A uniform brown covering wasformed. The solution was clear, its color had become still darker.

9:01 a. m. The sheet was wiped and showed to be perfectly clean afterthis treatment of about 6 minutes.

9:03 a.m. The sample-precipitate was brown. pH=2.3.

red-

9:07 a.m. A fresh sheet was introduced. The

temperature was lowered to 53 C. A uniform brown covering formed. 9:11a. m. The sheet was wiped and proved to be perfectly clean apart fromsmall spots. The solution was clear and brown colored. 9:15 a. m. Thesolution was heated to 70 C. It became turbid and the turbidity 4increased noticeably.- 9:18 a. m. The sample-precipitate was brown.

9:23 a.m. The sheet was introduced again. A

flocculent precipitate formed in the solution and a uniform covering onthe sheet.

Time of observation 9:25 a. m. The sheet was wiped andshowed to beperfectly clean after this treatment of about 7 minutes. The precipitatesettled down. 9:28 a. m. The sample-precipitate was dark brownolive-colored. pH=2.3. 9:50 a. m. The titratingcurve shown in Fig. 9

' was established.

The results of Test No. 8and 9 may be summarized as follows:

The acidstrength of 0.7% at the start caused in Test No. 8 a smallreduction of the time required for the pickling, while theclean-pickling eifect is appreciably reduced. The solution becameturbidionly at the end of the experiment, after the solution was keptabout one hour at a pH value of 2.3. In Test No. 9 the solution wasgiven suflicient time, at the beginning of the test, for the conversionof the ferrous compounds into ferric compounds; for this reason thecleanpickling eflect of the solution was satisfactory already with theflrst sample. The influence of the formation of colloids on the acidcontent of the solution could be ascertained in this test by figures.The influence is represented by a, distinct reduction of the pI-I valuefrom 2.3 to 1.7, whereby Equation No. I or the corresponding equation isconfirmed. The solution became turbid, and insoluble compounds wereseparated, when after the formation of colloids the pH value of thesolution had again increased to 2.3.

The influence of too high an acid strength at the start is shown by TestNo. 13, which may therefore also be detailed. In this test a solutioncontaining 2% HzSO4+2% NaNOa was used and the pickling operation wascarried out at 65 C.; the pH value at the beginning was thus 0.7.

Time of observation 9:30 a. m. The sample sheet was placed in thesolution. A very vigorous development of gas occurred. After half aminute the sheet was wiped and more than three fourths of the scaleshowed to have already been j could almost completely be wiped 9:41 a.m. The sample-precipitate was brown.

9:42 a. m. The sheet was again introduced.

The development of gas was moderate.

9:43 a. m. Thesheetwaswiped. Thescaleproved to be completely removedafter this treatment of about 2 minutes, but

the iron plate was again dull grey and not clean.

Time of observation 7 9:49 a. m. A fresh sheet was introduced into thesolution and again a vigorous development of gas occurred.

M 9:50 a. m. The sheet was wiped, the scale proved to be almostcompletely removed. 9:52 a.m. The sample-precipitate was brown. 9:53am.The sheet was again placed in position and again a vigorous developmentof gas occurred. 9:54 a.m. The sheet was wiped. The scale was not yetcompletely removed. 9:56 a. m. The sample-precipitate was redbrown. 9:57a.m. The sheet was again introducedinto the solution and a very vigorousdevelopment of gas occurred.

I 9:58 a. m. The sheet was wiped and, after this 10:10 a. m. The sheetwas again placed in the solution. When it was not moved, at some spots alight covering formed, which however disappeared on moving the sheet.

10:11 a.m. The sheet-was wiped. The scale showed to be not yetcompletely removed. The color of the solution was a lightgreenish-yellow.

10:12 a. m. The sample-precipitate" was redbrown. The pH value was nowincreased to 1.8. V

10:19 a. m. The sheet was again introduced and a vigorous development ofgas occurred.

10:20 a. m. The sheet was wiped. The scale showed to be not yetcompletely dissolved.

10:22 a. m. The sample-precipitate was redbrown.

10:26 a. m. The sheet was again introduced. A slight covering was formedon some spots.

10:27 a.m. The sheet was wiped and, after this treatment of about 4minutes, showed to be almost free from scale. Those spots which beforeshowed a covering; now appeared clean, while the remainder of the sheetwas grey.

10:32 a. m. The sample-precipitate was redbrown. A fresh sheet wasintroduced into the solutionand a light development of gas could beobserved.

10:85 aim. The sheet was wiped. Only some of the scale was removed.

. 10:39 a. m. The sheet was again introduced. On

moving the sheet a vigorous development of gas occurred and a coveringformed at some spots.

. 6. The greater the addition of sodium nitrate,

Time of observation 10:41 a. m. The sheet was ,wiped. The picklingeifect on the scale has very remarkably diminished, the sheet showed tobe clean only at some spots. 10:44 a. m. The sheet was again introduced.n

moving the sheet a vigorous development of gas occurred. A coveringformed. 10:46 a. m. The sheet was wiped and proved to be not yetperfectly free from scale after this treatment of about 7 minutes; thosespots which were free from scale, were however clean also. The titratingcurve of Fig. 10 was established.

Second day Time of observation 9:10 a. m. The sample-precipitate wasredbrown. pH=1.8. The solution was perfectly clear.

By the increased acid strength on starting used in Test No. 13 theoxidizing effect of the sodium nitrate was promoted. A clean-picklingeffect could not be observed at all at the beginning of the test and itoccurred only gradually, after the pH value was raised to above 1.8(H2804 less than 0.1). Now the picklingeifect on the scale showed to bevery remarkably diminished, so that the fifth sample sheet was not yetfree from scale after having been pickled 7 minutes. The titrating curveof Fig. 10 shows that a very, large quantity of dissolved ferriccompounds had formed in the solution. The. light color of the solutionhowever shows that no colloids were formed. At the beginning of the testcolloids could not be formed, since the acid strength was too high. Aformation of colloids could generally be observed only when the acidconcentration had dropped to less than 0.1% H2804. When thisconcentration was reached, the quantity, of iron compounds formed washowever already so large that the too high concentration of the ferriccompounds prevented the formation of colloids. Thus a starting acidstrength of 2% proved to be too high. p a

The experiments show under which conditions, in a pickling bath ofsulfuric acid and sodium nitrate, iron may be pickled to become clean,

while at the same time soluble iron compounds separate, so that thecontent of dissolved iron 'compounds in the solution does not exceed acertain limit defined by the hydrolytic equilibriurn. These conditions,which secure the desired result may be summarized as follows:

1. The solution must be warmer than 50 C.

2. The starting concentration of sulfuric acid must not exceed 1 percent.

3. The desired separation of insoluble iron compounds does not occurbefore the acid concentration has dropped to less than 0.1 per centH2804. f

4. The oxidizing agent present in the solution must sumce to prevent thepresence of any substantial quantity of ferrous compounds in .thesoiution,'since otherwise the content in dissolved iron compounds cannotbe prevented from increasing.

- 5. The clean-pickling effect also occurs only in the case that asumcient quantity of oxidizing agent is present.

the smaller is the pickling velocity.

.7. The content of sodium nitrate must be less than 5 per cent.

8. The starting acid strength of the solution should not be too low,since theoxldizing effect of the sodium nitrate decreases withdecreasing starting acid concentration.

If these conditions are fulfilled, the following 7 results may beobtained according to my invention:

The operation with the highly diluted solutions does not involve anydanger for the operator.

No hydrogen is combined with the metal, since in view of the oxidizingaction of the liquor, the acid, when acting on the metal, does not splitoff hydrogen in an obnoxious intensity.

In view of the equilibrium maintained in the solution, the bath is notenriched with metal salts. The solution may thereforebe used almostinfinitely without spoiling the product, and needs to be replenishedonly from time to time by an addition of acid or salt. When a solution'of nitric acid and ma nesium may for instance be replaced, without anundue increase in magnesium ions, by an addition to the bath of sulfuricacid; or an addition of alkali nitrate may be used to strengthen theoxidizing effect of the bath without increasing its acidcontent. alwaysbe taken that the admissible concentration of acid is not exceeded. Thisdoes not ine serves as pickling bath, the sulfate ions consumed in theprecipitating of the insoluble iron compounds When replenishing thebath, care mustvolve however any difilculty, since the basic precipitatepresent in the bath neutralizes any not too high excess of acid whichshould be added.

In the prior art the starting acid concentration is not distinguishedfrom the acid concentration maintained in the course of the picklingoperation. This difference is however important according to myinvention, since the desired result is only obtained by a very smallacid concentration during the pickling operation, while the startingacid concentration is preferably rendered high enough to secure that therequired quantity of ferric ions is formed in the bath.

By treating iron with a pickling bath in ac- .cordance with my inventiona pure metallic, perfectly clean surface is obtained, which issimultaneously passivated so that ,it will not be oxidized, when exposedto the free air, even after a long time. The surface is excellentlysuitable for receiving a protective coating of any kind.

The iron which is dissolved by the pickling action may be recovered inthe form of basic iron compounds and may be utilized as pigment.

The pickling time is rather long, if my process is directly applied tostrongly s'caled or rusted blanks. In this case I prefer to first freethe blank from scale and rust by any known means and thereupon toclean-pickle it in a pickling solution according to this invention, forinstance in an aqueous solution of 0.25% E04 and 2% potassium nitrate.

particularly iron with low carbon content which forms the material fromwhich sheet metal and other rolled products are made.

Various changes may be made in the details departing from the inventiondisclosed in the foregoing specification without or sacrificing theadvantages thereof. I claim:

1. The method of pickling iron, which comprises acting on the metal withan aqueous solution, heated to at-least 50 C., which contains freemineral acid in an amount sufliciently low to permitthe metal saltsformed to be precipitated by hydrolytic action, an oxidant, and up toabout 5 per cent of a salt, the anion of which difiers from that of theacid and the kation of which is less noble, in the sense of theelectromoti've series, than the metal to be treated.

2. The method of pickling iron, which comprises acting on .the metalwith an aqueous so- I lution, heated to atleast 50 0., which containsfree mineral acid in a concentration of not more than 1 per cent, anoxidant in an amount sumcient to oxidize substantially all the dissolvedions of the metal under treatment, and a salt, the anion of whichdiffers from that of the acid and the 'kation of which is less noble, inthe sense of the electromotive series, than the metal under treatment,allowing the acid concentration of said solution, while it is in contactwith the metal under treatment, to drop below 0.1per cent andmaintaining it below 0.1 per cent during the pickling treatment.

3. The method of claim 2, wherein sulfate and nitrate ions are presentin the solution.

4. The method of claim 2, wherein persulfate ions are added to thesolution which contains nitrate ions.

5. The method of pickling iron, which comprises acting on an ironsurface with an aqueous solution, heated to at least C., which containsup to 1 per cent sulfuric acid and up to about 5 per cent of an alkalimetal nitrate, allowing the acid concentration of said solution to dropduring its action on the metal under treatment, below 0.1 per cent andmaintaining it below 0.1 per cent for clean-pickling said metal.

6. 'I'he method of pickling iron, which comprises acting on iron with anaqueous solution, heated to at least 50 0., which contains free mineralacid in an amount suiflciently high to attack the iron under treatment,but sufficiently low to permit ferric compounds formed to be separatedin colloidal form by hydrolytic action, an electrolyte adapted tocoagulate colloidal ferric compounds present in the solution, and anoxidant dissolved in an amount sufliciently high to oxidizesubstantially all ferrous compounds present, but not-so high as toprevent the separation of coagulated ferric compounds formed.

7. The method-of claim 2, wherein the treatment is performed at atemperature ranging between and C.

8. The method of claim 2, wherein the metal to be pickled, before it istreated with the said solution, is superficially freed from scale andrust.

9. The method of claim 2, wherein a plurality of mineral acids ispresent in the solution.

10. The method of claim 2, wherein the solu tion contains a plurality ofsalts.

I WALTER HEIMIBERGER.

